1. Field of the Invention
When a starter is switched on in order to ensure the starting of the thermal engine of the vehicle, a substantial requirement for current arises which is close to the level of the short-circuit current of the starter, i.e. a current of approximately 1000 A. This requirement for current when the starter is switched on then decreases in intensity as the speed of the armature of the starter, corresponding to the rotor of the machine, increases.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
This initial current spike corresponds to a consequent drop in the voltage at the terminals of the battery. Other, less substantial voltage drops then occur during the starting phase, and correspond to passages through successive top dead centres of the thermal engine.
The development of so-called “reinforced” starters which are designed for automatic stop/start systems of the thermal engine (so-called “stop/start” or “stop and go” systems) now impose new constraints on car parts manufacturers, relative to the compliance with minimum voltage thresholds of the battery during the requirement for current when the starter is switched on. Thus, in their specifications, motor vehicle manufacturers define a first voltage threshold which is habitually contained between 7 and 9 V, below which the battery voltage must not descend. For the following voltage drops, corresponding to the top dead centres of the thermal engine, the battery voltage must remain higher than a second voltage threshold, which is habitually contained between 8 and 9 V. During the starting of the thermal engine, the voltage of the vehicle on-board network thus remains at a value which is sufficient to guarantee the required functioning of the parts of the vehicle.
The reinforced starters generally have a power level higher than conventional starters, so as to obtain rapid starting for increased comfort of the users. This results in a higher requirement for power when switching on takes place, and thus to a first drop of the battery voltage which goes beyond the habitual values, in relation to high demands. This creates a genuine difficulty for the designer, since, in order to be at a higher battery voltage, the starter would have to have internal voltage drops which were so great that it would no longer then have the power necessary to drive the thermal engine at a sufficient speed, at a low temperature.
In the prior art, solutions have been proposed to the above-described problem. A first known solution by the inventive body is based on the use of electronic converters for stepping up the voltage, in order to prevent a voltage level which is too low in the on-board network. A major disadvantage of these converters consists in the substantial costs which they introduce.
Another known solution proposes controlling the starter by means of two relays, timing, and a current-limitation resistor. In a first functioning phase, the duration of which is determined by the timing, an additional resistor is inserted in series in the starter circuit, and limits the initial current spike. In a second functioning phase, the additional resistor is taken out of the starter circuit in order to permit the passage of a sufficient current in the armature of the starter, and to allow an increase in the speed of the latter.
Documents EP2080897A2 and EP2128426A2 describe a starter of the above type. Apart from the disadvantage of the extra cost which the additional control relay, the timing and the current-limitation resistor of this additional relay involve, the introduction of this additional relay, comprising mobile mechanical parts which are subject to wear, has a negative impact on the resistance of the starter in terms of the number of starting cycles which the starter must be able to withstand without hindrance for the starter. The resistance of the starter in terms of the number of the starting cycles is a particularly severe constraint for starters which are designed for stop/start systems. In fact, starters of this type must withstand approximately 300,000 starting cycles, i.e. ten times more than the approximately 30,000 cycles required from the conventional starters.
In addition to the above-described disadvantages, the use of this second solution according to the prior art can prove to be inappropriate when compliance with a voltage range which is restrictive in terms of time is required by the motor vehicle manufacturer. A range of this type generally comprises a low voltage threshold corresponding to the first voltage threshold indicated above, and a high voltage threshold corresponding to the second voltage threshold. A rising voltage gradient is also provided in the range, between the low threshold and the high threshold.
The tests carried out by the inventive body, with the usual values of the manufacturers for the duration of the low threshold and the slope of the gradient of the range, show the difficulty which exists, with this second solution according to the prior art, of remaining within the range. In fact, it has been found that there is a risk of going outside the range at the level of its voltage gradient, when the battery voltage, after having been rectified once the initial current spike has been absorbed, drops again at the end of the timing, with the current passing through the armature of the starter then increasing substantially, because of the removal of the resistor for limitation of the current of the starter circuit. After thus going outside the range, the battery voltage may remain below the range for a certain period of time, and come back into the range only after the end of the rising voltage gradient, whereas the instant of the start of the high voltage threshold has already been reached.